A typical flexible flowline consists of several distinct concentric layers which give strength and flexibility to the flowline. The innermost layer, known as the carcass, prevents collapse of the flowline due to external hydrostatic pressure. The next layer, known as the barrier, is made from a synthetic polymer material and serves to prevent leakage of the contents from the flowline. There may then be a succession of other layers of varying materials.
A flexible flowline may be used, for example, as a dynamic riser to couple a rigid flowline or another flexible flowline on the seabed to a floating vessel or buoy to convey production fluids such as oil, gas or oil/gas mixtures under pressure from an oil gas well or platform to the vessel or buoy. Hence, an end fitting is required to couple the flexible flowline at each end to the adjacent flowline or wellhead and the vessel or buoy.
A leak-tight seal in the end fitting has sometimes been obtained in the prior art with an inner seal ring which engages with the external surface of the barrier. This seal ring also holds the barrier and carcass in position in the end fitting by the frictional force generated by the pressure of the seal ring on the barrier layer.
However, problems may arise with this conventional type of end fitting where the flowline and end fitting are subjected to periodic heating and cooling. Periodic heating and cooling occurs when the flowline and end fitting are subjected to repeated changes in temperature. For example, the flowline may be exposed to temperatures in excess of 100.degree. C. when production fluid is conveyed through the flowline and subsequently may be exposed to temperatures dose to 0.degree. C. from seawater when flow of the production fluid ceases.
Repeated thermal cycling can result in failure of the carcass and barrier to maintain their original shape. During heating, loss of plasticizer may occur in the barrier which may cause shrinkage of the barrier. During cooling, the barrier tends to shrink axially due to thermal contraction.
Due to shrinkage of the barrier, the inner seal ring may no longer exert enough pressure to hold the barrier and the carcass to the end fitting. As a result, the barrier and the carcass may slip out of the inner seal ring, thus causing leakage and failure of the connection between the end fitting and the flowline.